This invention relates generally to fiber-optic sensors and more particularly to fiber-optic colorimetric sensors.
Colorimetric fiber-optic sensors are known to the art. For example, pH and PCO2 sensors are described in the articles "Fiber-Optic pH Probe for Physiological Use", by Peterson et al, published in Analytical Chemistry, Vol. 52, pp. 864-869, 1980, and "A Fiber-Optic PCO2 Sensor", by Vorek et al, published in Annals of Biomedical Engineering, Vol. 11, pp. 499-510, 1983. Both sensors employ optical fibers to deliver light to and carry light from a sensor head which contains a phenol red based dye system. In both sensors, light absorption properties of phenol red are utilized in order to produce a color change which can be monitored optically. Fiber-optic sensors employing phenol red based dye systems are also disclosed in U.S. patent application Ser. No. 07/314,615, for "BLOOD GAS MONITORING SENSORS", by Schweitzer and Proctor, filed Feb. 23, 1989, now U.S. Pat. No. 5,047,208, and incorporated herein by reference in its entirety.
Phenol red is a weakly ionizing acid which disassociates into an acid form having an absorption peak at about 440 nm and a base form having an absorption peak at about 570 nm. The proportions of the acid and base forms are determined by the pH of the solution containing the dye. Therefore, the pH of the solution containing phenol red can be monitored using light from a green LED having a frequency band centered at or about 570 nm. Absorption at a second wavelength is also measured in order to compensate for changes in the overall performance of the sensor. Typically, the second wavelength employed is in the infrared range, at which the absorptive properties of phenol red are substantially the same whether in the acid or base form.
The light absorption properties of phenol red in the green wavelengths are also affected somewhat by temperature. Thus, the accuracy of pCO.sub.2 or pH measurements made using sensors according to the prior art if a separate temperature sensor is available. However, this complicates the measurement process and requires the use of additional equipment.